We describe a system of two groups of multiwire proportional chambers and its special purpose interface used in the Single Arm Spectrometer (SAS) facility at Fermilab. The first set measures the coordinates of the fast particle going into the spectrometer arm with a space resolution down to +/-0.5 mm. The second set, close to the target, measures the multiplicity and angular distributions of charged particles coming from the interaction. The chambers of this set have three sensitive planes with wires at 120 degrees in the same gas volume to measure three coordinates for each physical point, two readout channels per wire to minimize the inefficiency due to the very high flux and insensitive regions at the center of the chambers where they are crossed by the beam.